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Agricultural soil C sinks, potentially significant depository of atmospheric CO2, are important to numerous ecosystems services including mitigation and adaptation to climate change, improvements in quantity and quality of renewable fresh water resources, enhancement of biodiversity, and increase in agronomic productivity. Technical soil C sequestration potential depends on C lost during the prior land use, soil management, and the attendant degradation in soil quality. The global maximum soil C sink capacity is 78±12 Gt C, with the technical potential of C sequestration rates of 0.6 to 1.2 Gt C/yr in cropland coils, 0.14-1.0 Gt C/yr in restoring salt-affected soils, 0.6-1.7 Gt C/yr through desertification control, 0.8-1.0 Gt C/yr through afforestation in tropical forest ecosystems, and 0.3-0.5 Gt C/yr in the savanna ecosystems. Thus, the global potential of C sequestration in the terrestrial biosphere is 2.5-5.3 Gt C/yr. The most cost effective technologies are: conservation tillage and residue management, and improved agronomy practices, nutrient management, and rice management. Rates of soil C sequestration can be enhanced by improving the eco-efficiency of inputs used in agroecosystems (e.g., fertilizers, irrigation). Improvement in soil quality, by increasing soil organic C pool by 1 Mg/ha/yr in the root zone, can increase food production in developing countries by 32 million Mg/yr of grains and 9 million Mg/yr of roots and tubers. Adoption of conversation tillage and other recommended management practices can be promoted among resource-poor farmers by payments for ecosystem services; however, the price of soil C must be determined by fair, just and transparent criteria.